This invention relates to corona treatment of materials, and in particular to corona treatment of elongated objects, such as wire, which has at least a thermoplastic or polymer skin.
Corona modification of the surface properties of thermoplastics and other polymers is well known. Treatment of the surface properties is used in order to increase the surface tension, and hence the wettability of common polymers, such as fluoronated polyolefins, which have quite slick surfaces and therefore do not readily accommodate paint, print or other coatings.
Traditional corona surface treatment incorporates a generator and transformer for creating low frequency, high voltage power. That power is then directed to an electrode which is spaced from a second, grounded electrode. The material to be treated is placed or passed between the spaced electrodes for a predetermined amount of time, depending on the material, the spacing between the electrodes, the voltage applied and the frequency.
High frequency surface treating has been introduced by the assignee of this application. By high frequency, it is meant above 1 MHz, preferably around 2 MHz. At such high frequencies, a useful corona is produced using only a single electrode, thus eliminating the need for the second grounded electrode.
In the past, when continuous treatment of lengthy materials, such as wire, is desired, the wire is passed through a loop electrode for treatment. However, because of the small width (in the axial direction of the wire) of the loop, treatment must proceed rather slowly. The power applied can only be increased to certain limits, however, since direct electrical discharge between the electrode and the wire can occur, causing destruction of the insulated coating covering the wire. In order to alleviate this problem and increase somewhat the speed of treatment, a metal tube has been substituted for the loop electrode. However, the same discharge problems occur, thus not permitting high speed treatment of coated electrical wires.